1. Field of the Invention
This invention relates to digital communication systems and, more particularly, to a detection system and method for an Ethernet receiver.
2. Discussion of Related Art
The dramatic increase in desktop computing power driven by intranet-based operations and the increased demand for time-sensitive delivery between users has spurred development of high-speed Ethernet LANs. 100BASE-TX Ethernet, using existing category-5 copper wire, and the newly developed 1000BASE-T Ethernet for Gigabit/s transfer of data over category-5 copper wire require new techniques in high speed symbol processing. Gigabit per second transfer can be accomplished utilizing four twisted pairs and a 125 megasymbol/s transfer rate on each pair where each symbol represents two bits.
Physically, data is transferred using a set of voltages where each voltage represents one or more bits of data. Each voltage in the set is referred to as a symbol and the whole set of voltages is referred to as a symbol alphabet. In gigabit transfer, for example, data is usually sent with a set of five voltage levels (PAM-5), each symbol representing two (2) bits.
One system of transferring data at high rates is Non Return to Zero (NRZ) signaling. In NRZ, the symbol alphabet {A} is {−1, +1}. A logical “1” is transmitted as a positive voltage while a logical “0” is transmitted as a negative voltage. At 125 M symbols/s, the symbol rate required for Gigabit transfer over four category-5 wires, the pulse width of each symbol is 8 ns.
Another example of a modulation method for high speed symbol transfer is MLT3 and involves a three level system. (See American National Standard Information system, Fibre Distributed Data Interface (FDDI)—Part: Token Ring Twisted Pair Physical-Layer Medium Dependent (TP-PMD), ANSI X3.263:199X). The symbol alphabet for MLT3 is {A}={−1, 0, +1}. In MLT3 transmission, a logical 1 is transmitted by either a −1 or a +1 while a logic 0 is transmitted as a 0. A transmission of two consecutive logic “1”s does not require the system to pass through zero in the transition. A transmission of the logical sequence (“1”, “0”, “1”) would result in transmission of the symbols (+1, 0, −1) or (−1, 0, +1), depending on the symbols transmitted prior to this sequence. If the symbol transmitted immediately prior to the sequence was a +1, then the symbols (+1, 0, −1) are transmitted. If the symbol transmitted before this sequence was a −1, then the symbols (−1, 0, +1) are transmitted. If the symbol transmitted immediately before this sequence was a 0, then the first symbol of the sequence transmitted will be a +1 if the previous logical “1” was transmitted as a −1 and will be a −1 if the previous logical “1” was transmitted as a +1.
The detection system in the MLT3 standard needs to distinguish between 3 voltage levels, instead of two voltage levels in a more typical two level system. The signal to noise ratio required to achieve a particular bit error rate is higher for MLT3 signaling than for two level systems. The advantage of the MLT3 system, however, is that the energy spectrum of the emitted radiation from the MLT3 system is concentrated at lower frequencies and therefore more easily meets FCC radiation emission standards for transmission over twisted pair cables. Other communication systems may use a symbol alphabet having more than two voltage levels in the physical layer in order to transmit multiple bits of data using each individual symbol.
In Gigabit Ethernet over twisted pair Category-5 cabling, for example, PAM-5 data can be transmitted over four twisted copper pair at an individual twisted pair baud rate of 125 Mbaud. In PAM-5, data is sent with 5 voltage levels, designated as symbol alphabet {A}={−2, −1, 0, 1, 2} although the actual voltage levels may be different from those. Each symbol, therefore, can be used to code more than one bit of data.
Any other modulation scheme for symbol coding can be utilized, including quadrature amplitude modulation (QAM). In QAM schemes, for example, the symbols are arranged on a two-dimensional (real and imaginary) symbol constellations (instead of the one-dimension constellations of the PAM-5 or MLT-3 symbol alphabets).
There is a need for transmitters and receivers for receiving transmission over multiple twisted copper pair using larger symbol alphabets (i.e., 3 or more symbols). There is also a need for transceiver (transmitter/receiver) systems that, while operating at high symbol rates, have low bit error rates.